The present invention relates to the field of exhaust air systems for buildings and/or other enclosed areas, and more particularly, to exhaust discharge nozzles configured to be attached to the outlets of exhaust fans, exhaust ducts and/or stacks, and similar exhaust type equipment/devices and are specifically designed to be installed in the outdoor ambient.
Many commercial and industrial processes exist which introduce hazardous and/or noxious chemicals into the building exhaust. These chemicals originate from a host of commercial/industrial processes within critical environments such as research laboratories, chemical storage facilities, generator housing rooms, thermal oxidizers, exhaust chemical scrubbers, etc. It is of paramount importance that the proper precautions are taken to ensure that the effluent is effectively managed 100% of the time. Specifically designed, purpose built exhaust systems are required to mitigate hazardous concentrations of processes chemicals. As governed by the ASHRAE 2011 HVAC Applications Handbook, a comprehensive flow model of the building must be executed to determine critical fluid flow patterns based on the unique geometry and wind flow patterns for the site. Consideration for the location of near-by air building fresh air intakes is a critical factor which must be accounted for so as to avoid possible effluent re-entrainment into the facility in unprocessed concentrations. In order to be effective, the critical exhaust provisions must be properly designed and must achieve continuous rated performance in the real world dynamic environment where the system is to operate. Failure to meet any of the above criteria would jeopardize the safety of those working in and around the proximity of the critical environment and/or residents of surrounding communities.
An effective solution, as standardized by ASHRAE, is to propel exhaust gases upward to a critical height above the building roofline where the atmospheric free stream can provide sufficient plume dilution, thus reducing the concentrations of hazardous chemicals to levels deemed safe. This critical height is termed the “effective stack height.” In its simplest form, the effective stack height is the height at which a theoretical centerline of the building exhaust plume becomes completely horizontal due to the impact of the specified horizontal cross wind velocity. The effective stack height, hse (ft), can be calculated from the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) HVAC Applications Handbook as:hse=hs+hr−hd Where:
hs is the physical exhaust system height (ft)
hr is the plume rise (ft)
hd is the amount of stack wake downwash in (ft)
The plume rise component, hr, is the distance the exhaust plume will be propelled above the terminal discharge point of the physical equipment. Plume rise for momentum driven flow is calculated based on the recommendations of the ASHRAE. From as early as 1999 through 2010 the ASHRAE HVAC Handbook has stipulated the use of a special case of the Brigg's Equations to determine plume rise hr, which is defined as:hr=3.0de(Ve/UH)                de is the effective (hydraulic) diameter (ft) at the terminal discharge point of the system computed from: de=(4Ae/π)^(½), where Ae is the cross-sectional area of the discharge opening        Ve is the equipment exit velocity (ft/min) at cross wind velocity        UH is the cross wind velocity (ft/min) at the building rooflineThis adaptation of the Briggs Equation is a function of dynamic variables. Equipment performance data must be acquired using dynamic testing parameters. Specifically, the equipment exit velocity, Ve, must be measured with the specified design cross wind, UH, applied to the system. Moreover, it is a necessary condition that the effective diameter, de, be determined for the location where the equipment exit velocity, Ve, was measured. It is recommended that this location be final discharge point (i.e. terminal location) of the exhaust system to the atmosphere. For this form of the Briggs equation for plume rise to be applicable, the discharge velocity profile at the system discharge must be characterized as uniform. A uniform velocity profile is defined as having minimal velocity gradients in the transverse plane of system discharge.        
The initial adaptation and application of the Briggs equation for plume rise did not effectively capture many critical site specific parameters, and the accepted method for calculating plume rise has been redefined in the ASHRAE 2011 HVAC Applications handbook using the Briggs equation for the vertical jet momentum of the exhaust versus downwind distance as:hr=min{βhx, βhf}
β is the stack capping factor, 1.0 without cap as in the present invention
The plume rise verses downwind distance hx in (ft) is obtained from:hx=[(3Fmx)/(βj2UH2)]^(⅓)
Fm is the momentum flux (ft4/s2) and is calculated as Fm=Ve2(de2/4)
βj is the jet entrainment coefficient computed as βj=⅓+(UH/Ve)
x is the downwind distance
The final plume rise hf in (ft) is determined from:hf={0.9[Fm(UH/U*)]^(½)}/(UHβj)
UH/U* is the he logarithmic wind profile computed as UH/U*=2.5 ln(H/z0)                H is the building height above ground level (ft)        U* is the friction velocity (ft)        z0 is the surface roughness length (ft) which can be obtained from the Atmospheric Boundary Layer Parameters Table in Chapter 45 of the ASHRAE 2011 HVAC Design Handbook.The possibility of stack wake downwash, hd, is an essential component to evaluate when computing the effective stack height of an exhaust system. Stack wake downwash occurs where low velocity exhaust streams are pulled downward by negative pressures immediately downstream of the exhaust system discharge. The amount of stack wake downwash in (ft) can be obtained from hd=de[3.0−β(Ve/UH)]        
As specified in the ASHRAE 2011 standard, the cross wind velocity at the building roofline UH, as applied to all equations which require this parameter, is the maximum design wind speed at the building roof height at which air intake contamination must be avoided. As stated by ASHRAE, this maximum design speed must be at least as large as the hourly wind speed exceeded 1% of the time. Chapter 14 of the 2009 ASHRAE Fundamentals Handbook lists this value for many cities.
Upon examination of the equation for effective stack height it becomes evident that the most critical parameters affecting a system's ability to achieve this specification are discharge geometry (de), discharge velocity (Ve), and the design wind speed (UH) where the system is to operate. Furthermore, the American National Standards Institute/American Industrial Hygiene Association ANSI/AIHA Z9.5 2012 Laboratory Ventilation standard mandates a minimum discharge velocity of 3000 ft/min be constantly maintained in order to be in compliance. Standard Z9.5 2012 also specifies that the physical exhaust system height, hs, be a minimum of 10 ft. above adjacent roof lines and air intakes and in a vertical up direction.
It should be noted that standard industry testing methods, at the present time do not incorporate cross winds into the testing protocol. The Air Movement and Control Association (AMCA) has developed AMCA Standard 260-07 Laboratory Methods of Testing Induced Flow Fans for Rating and is generally accepted as the industry standard. However, while this test does certify discharge flow volume of an induction exhaust system, it does not include dynamic testing with the influence of a cross wind. Therefore, using outlet flow data to calculate system exit velocities measured according to AMCA standard 260-07 can lead to erroneous discharge velocity ratings. Furthermore, if static system exit velocities (i.e. no cross wind present during measurement) are used in the special case Briggs Equation, which is a function of dynamic variables only, to determine plume rise, the prediction of performance will be physically incorrect. Plume rise (i.e. the quotient) determined in this manner would always be mathematically undefined (i.e. infinite asymptote) due to the 0 ft/min cross wind velocity devisor; which is an impossible physical phenomena to achieve. However, if the AMCA standard 260-07 were modified to incorporate cross wind, then the Briggs equations would be a mathematically valid method of calculating plume rise, provided that the velocity profile at the discharge was uniform. Additionally, an advanced engineering approach is to use computational fluid dynamics (CFD) software to calculate system performance; the AMCA 260 test can be simulated with the cross wind component included to develop real world performance data. The Briggs equation is valid for calculating plume rise using the CFD data; however this only applies to systems with a uniform discharge profile. Additionally, the most current methodology of calculating plume rise as defined by ASHRAE should always be used.
Complying with the necessary laws, codes, standards and recommendations is becoming increasingly challenging, as recent advancements have led to an increasing number of variable volume laboratory designs and installations. One of the most significant benefits of variable volume systems is the ability to turn down the exhaust air volume in response to usage requirements. This reduction is exhaust flow results in a significant energy savings. However, reducing the exhaust flow volume using conventional/existing technology has historically made achieving the required effective stack height and minimum exhaust discharge velocities challenging due to the accompanying reduction in discharge velocity.
The present invention is designed to instantaneously modulate and control discharge geometry and discharge velocity in response to varying primary exhaust air flows, dilution requirements, and roof line wind speeds, so that the mandated effective stack height is continuously achieved. The device is designed with a variable discharge diameter to gradually accelerate the exhaust effluent to a sufficiently high velocity. An adjustable impingement pod, which runs the full length of the nozzle section, provides a mechanism to gradually reduce the nozzle's cross-sectional area, thereby producing a uniform acceleration of the primary exhaust stream. The uniform acceleration has the specific benefit of minimizing high velocity gradients within the nozzle which contribute to exhaust stream energy loss. A unique controls strategy is employed which provides on demand response to varying primary flow conditions, dilution and changing roofline wind speeds. Bypass air dampers, system discharge area and motor speed adjust in a coordinated effort to meet operational exhaust requirements as outlined above. Thus, the present invention is an energy efficient alternative to conventional technology.
The application of discharge nozzles at the exit point of exhaust systems enhances the performance capability with the specific intent of maximizing the exhaust/effluent dispersion into the upper atmosphere of the hazardous contaminated air and/or effluent gases and vapors from buildings, rooms, and other enclosed spaces. Discharge nozzles able to provide a superior alternative to conventional tall exhaust stacks which are costly to construct and are visually unattractive by today's standards. Properly designed nozzles are capable of propelling high velocity plumes of exhaust gases to heights sufficient to prevent stack wake downwash and disperse the effluent over a large upper atmospheric area so as to avoid exhaust contaminant re-entrainment into building ventilation intake zones.
A further development of the variable-volume exhaust nozzle design is the type nozzle that employs the Venturi effect to draw additional ambient air into the primary effluent stream. The venturi type nozzle can further be described as an aspirating, or induction type, as related to conventional technological description for this type nozzle. The additional induced air volume dilutes the primary exhaust gases at/near the nozzle as the combined mixed air volumes are released into the atmosphere. Also, with this exhaust-air mixture volume increase, the discharged gas is expelled at a higher velocity, achieving a greater plume height. The underlying effect of greater volume at greater discharge velocity is an increased effluent momentum, which assists with the effluent disbursement into the atmosphere.
The features and functions of induction nozzles are described in greater detail in U.S. patent application Ser. No. 13/067,269, the disclosure of which is incorporated herein by reference.
High plume lift is particularly critical with regard to exhaust gases from potentially contaminated sources, such as laboratories and other facilities in which chemical processes produce noxious fumes. To insure that potentially contaminated exhaust reaches a minimum altitude to avoid downwash, many environmental and building code standards specify a minimum discharge velocity from an exhaust nozzle. For example, ANSI Z9.5 2012 currently requires a minimum discharge velocity of 3000 feet per minute (FPM) at the outlet of a lab exhaust nozzle.
Maintaining a minimum exhaust nozzle discharge velocity can be problematic when there is a high turndown ratio in the critical space, meaning the primary exhaust flow rate is highly variable. This is typically the case in laboratories, for example, where some of the fume hoods may be inactive at any given time, so that the primary exhaust rate is often below the design value for the exhaust fan. Since lowering the fan speed can reduce the exhaust outlet velocity below the 3000 FPM minimum, the conventional approach in the past has been to maintain a constant fan speed while opening bypass air dampers to draw in ambient air.
One existing approach to variable primary flows is to select fans to perform at the maximum exhaust flow condition of the critical space. When the flow requirements of the critical space are reduced, a bypass damper is opened to incorporate unconditioned outside air into the fan to make up the difference in flow volume. While this approach is functional, the practice of running exhaust fans continuously at speeds designed to handle the maximum design exhaust flow condition is wasteful in terms of energy consumption. To conserve energy, it's preferable to use variable speed fans in which the fan speed decreases as the primary exhaust flow rate decreases. In order to maintain a minimum discharge velocity through an exhaust nozzle, the cross-sectional outlet area of the nozzle can be varied by mechanical means, such as dampers.
Mechanical variation of the nozzle outlet area has the disadvantage of causing non-uniform exhaust flow gradients at the approach to the constricted nozzle opening. In other words, the exhaust flow velocity does not increase uniformly with respect to distance travelled. This creates the opportunity for turbulent flow pattern to develop and produces non-uniform pressure and velocity profiles at the constricted nozzle outlet. As a result, an uncorrected Briggs equation for calculating plume rise would not apply to such a device, and performance would not be readily predictable.
As discussed in U.S. patent application Ser. No. 13/067,269, the use of a wind band at the nozzle outlet provides the advantages of shielding the exhaust discharge from cross-winds, which reduce plume height, as well as inducing ambient air flow through the windband, thereby increasing discharge flow volume and velocity. But effective induction through the windband requires uniform pressure and velocity profiles at the nozzle outlet, which cannot be achieved if the nozzle outlet is mechanically constricted.
Instead of mechanically constricting the nozzle outlet area in response to reduced fan speed, the present invention uses an axially-extendable, upwardly tapered flow-impinging pod within the nozzle to create a variable annular nozzle outlet opening. As the impinger pod is extended upward through the nozzle opening, the annular space around the pod narrows gradually and uniformly in the direction of the nozzle outlet, thereby enabling a linear velocity gradient and producing a uniform discharge velocity profile conducive to optimal induction through the windband, as well as maximizing the integrity of the exhaust plume